I. Field of the Invention
The present invention relates to a throttle valve control apparatus for an engine and, more particularly, to a throttle valve control apparatus for an engine wherein an air-fuel ratio of a gas mixture supplied to the engine is changed, and a throttle valve disposed in an intake passage is electromagnetically controlled.
II. Description of the Prior Art
In most engines, especially internal combustion engines designed for use on automobiles, requiring various operating states, air-fuel ratios have been controlled and changed according to predetermined conditions in recent years. The air-fuel ratio is set to be large (i.e., a lean mixture) during cruising under a small load requiring no high output, while the air-fuel ratio is set to be small, i.e., a rich mixture, during cruising under a large load requiring a high output.
In Otto engines using gasoline, LPG, alcohol, etc., as fuel, a throttle valve is pivotally disposed in an intake passage to control the amount of a gas mixture by adjusting a throttle opening. In general, the throttle valve is mechanically coupled to an accelerator operated by a driver through a linkage such as a wire. In other words, an accelerator position is proportional to a throttle opening so that the throttle opening can be determined solely as a function of the accelerator position. The term "accelerator position" referred to in this specification means a position of an accelerator or an amount operated thereby, relative to the position thereof at which or in the amount in which the accelerator is not operated, expressed in percentage, and it is given as 0% when the accelerator is not operated and as 100% when it is operated to the maximum extent. The term "throttle opening" referred to herein means a ratio of a given angular position of a throttle valve to an angular position of the fully open throttle valve if the fully closed position of the throttle valve is given as 0% and the fully open position of the throttle valve is given as 100%.
U.S. Pat. Nos. 4,112,885 and 4,168,679 describe electromagnetic throttle valve control apparatuses in place of mechanical ones. In an electromagnetic apparatus, an accelerator pedal is electromagnetically coupled to a corresponding throttle valve. More specifically, U.S. Pat. No. 4,112,885 discloses an arrangement for optimally controlling a throttle valve using a reversible drive motor according to accelerator positions and throttle openings, while the throttle valve is driven by the reversible drive motor.
In the electromagnetic control apparatus described in U.S. Pat. No. 4,112,885, a fuel injection quantity is determined according to the accelerator position, and a target amount of intake air is determined according to the determined fuel injection quantity, thereby controlling the throttle opening so as to obtain the target amount of intake air. In addition, the fuel injection quantity and an engine speed are used as parameters for determining the target amount of intake air. Further, an air-fuel sensor detecting an air-fuel ratio in the exhaust gas is used to control the throttle opening, i.e., an amount of intake air by feeding back the detected air-fuel ratio so as to obtain a desired air-fuel ratio.
If output changes are to be caused by changes in air-fuel ratio, an output at a small air-fuel ratio (the rich mixture) is higher than that at a large air-fuel ratio (the lean mixture). In a conventional engine of the type of which the air-fuel ratio is changed according to predetermined operating conditions, considerably large variations in output may be caused upon changes in air-fuel ratio. In other words, even if the driver keeps the opening of the accelerator, i.e., accelerator position, at an identical level, the engine output varies upon changes in air-fuel ratio. Therefore, output variations cause the driver discomfort. This feeling of discomfort that may be caused by variations in air-fuel ratio may occur as difference in the feeling of engine power even during the traveling after changes of the air-fuel ratios, and this tendency is likely to be remarkable when the engine is accelerated, in particular when it is started.